The production of acidic drainage from reactive sulphidic tailings and waste rock deposition produced during mining and milling of sulphide containing ores is a major ecological problem. This acid mine drainage can have devastating effects on receiving waters.
Acid mine drainage is the result of both the biotic and abiotic oxidation of sulfide minerals typically found in tailings ponds and waste rock dumps. The acid mine drainage typically has a low pH, high gulfate content and contains high concentrations of dissolved metals which can be detrimental to the biota in receiving waters.
The processes involved in the initiation of acid generation are not well understood. The autotrophic thiobacilli are capable of oxidizing a number of metal gulfides to produce gulfuric acid and solubilize metals. Some researchers have suggested that a succession of pH-dependent microbial activities was responsible for establishing the acidic conditions required for growth of the sulfur- and iron-oxidizing bacteria responsible for the production of acid mine drainage. It has been postulated that acidification of pyrite occurs in three stages and that both biotic and abiotic reactions occur in the first stage with the abiotic processes predominating (above pH 4.5). During the last two stages (below 4.5) biological reactions predominate. The most well characterized of the acidophilic thiobacilli is Thiobacillus ferrooxidans which has a pH range for growth between 1.0 and 4.0. At pH values lower than 4.5, iron oxidizing bacteria such as T. ferrooxidans catalyze the oxidation of ferrous iron in both the soluble and the mineral phase to ferric iron. At pH 3.0 the bacterial oxidation of ferrous iron is 10.sup.6 times more rapid than the chemical oxidation.
Although the role of microbial activity in initiating the oxidation of tailings was previously not considered to be important, more recent research has shown that bacteria are involved in the initial oxidation stage. A number of Thiobacillus species are capable of rapid growth at neutral pH values commonly found in fresh tailings. Thiobacillus thioparus is active over a pH range of 6.0 to 8.0 and can oxidize a number of reduced sulfur compounds such as elemental sulfur, sulfide, sulfite, thiosulfate and thiocyanate. Thiobacillus novellus is active over the pH range of 6.0 to 8.0 and can grow on either organic or inorganic substrates.
Amelioration of acid mine drainage can be accomplished by either preventing the generation of acid mine drainage or treating the effluent after it has been produced. Techniques for the prevention of acid mine drainage include the use of both wet and dry covers. Another approach has been to use bacterial inhibitors active against T. ferrooxidans, such as acrolein as described in Bockowski et al, U.S. Pat. No. 5,171,454. At low pH values artionic surfactants are bactericidal. Surfactants alone have a limited lifetime due to dilution and leaching. Slow-release rubber-based surfactant pellets that should extend the effective lifetime of these agents have been developed. More recently, longer term field data has shown that the use of inhibitors significantly reduced the generation of acid mine drainage in materials such as overburden, coal refuse and metal mine waste rock. Another approach would be to use compounds that inhibit the bacteria involved in the initial phases of the oxidation of tailings. The inhibitors would prevent oxidation during the period that they are exposed to oxygen until a wet or dry cover can be applied.
Thiol-binding reagents such as N-ethyl maleimide and iodoacetamide have been shown to block the oxidation of thiosulfate to sulfate by Thiobacillus neapolitanus. Glutathione is involved in the oxidation of thiosulfate by other species of thiobacilli via the formation of glutathione polysulfides which are subsequently oxidized to produce sulfite. The oxidation of elemental sulfur by Thiobacillus thiooxidans can be blocked by the addition of N-ethyl maleimide. Thus, numerous studies have shown the involvement of thiol groups in the oxidation of sulphur and thiosulfate by various thiobacilli. The use of thiol blocking reagents to inhibit the initial steps of sulfur oxidation that occur at higher pH values is another potential strategy for the prevention of acid mine drainage. However, many of the thiol blocking compounds such as N-ethyl maleimide are relatively toxic.
U.S. Pat. Nos. 4,306,988 and 4,329,495 describe two 1,3,4-thiadiazole derivatives that are effective corrosion inhibitors in acid metal-treating baths. However, the two patents describe the 1,3,4-thiadiazoles in the form of poly(oxyalkylated) derivatives. These derivatives are described as having low toxicity, but are not effective for inhibiting the formation of acid mine drainage.
It is the object of the present invention to find compounds which are capable of inhibiting the formation of acid mine drainage while also having low toxicity.